In many chemical reactions, for example the phosgenation of diamines for the preparation of isocyanates for polyurethane synthesis, or in vinyl chloride production, HCl is obtained in large amounts as a byproduct. There are various possibilities for utilizing the HCl obtained: marketing, further processing, for example in oxychlorination, disposal by neutralization, use for the preparation of inorganic chlorides or recycling to give chlorine. Since the trend in the market for HCl is difficult to assess, and in view of a steadily increasing amount of HCl from production processes, there is a considerable demand for HCl recycling processes in which chlorine is recovered as the desired substance.
An established recycling process is, for example, electrolysis of HCl or CuCl.sub.2 by the Westvaco process, as described by J. Gordon, Chem. Eng. (1953), 187; and Anon, Chem. Eng. (1960), 63. However, the high energy costs are the disadvantage aspect of this process.
The prior art long ago disclosed various processes for the preparation of chlorine by HCl oxidation. The oldest is the process developed by Deacon in 1868 for the direct and continuous reaction of HCl with air or oxygen over a copper chloride catalyst (cf. for example the review article by Kepinski, J., Tilly, J., Katucki, K. in Przem. Chem. 57(1) (1978), 14-17 and Kepinski, J., Kalucki, K. in Szczecin. Tow. Nauk., Wydz. Nauk. Mat. Tech. 9, 1973, 37-49). The reaction is equilibrium-limited so that the conversion is not more than 75%. The product stream thus also contains HCl, H.sub.2 O and air/oxygen in addition to Cl.sub.2. This necessitates subsequent, expensive working up of the Cl.sub.2 present, serious corrosion problems occurring owing to the aqueous hydrochloric acid present in the product gas.
A modified, large-scale industrial process is the Shell-Deacon process, in which HCl is oxidized with air to give chlorine by heterogeneous catalysis over a supported CuCl.sub.2 /KCl/LaCl.sub.3 catalyst in a fluidized-bed reactor with a yield of about 77%. This process is described, for example, by J. Th. Quant et al. in The Chemical Engineer, July/August 1963, page 224.
A further process which was previously used industrially is the Weldon process, in which manganese dioxide is used for oxidizing HCl. However, the chlorine yield was only about 30% since half the HCl used was lost in the form of CaCl.sub.2 during the recycling of the MnCl.sub.2 to MnO.sub.2 with the use of Ca(OH).sub.2 and O.sub.2.
In the Kel chlorine process from Kellog, HCl is converted into chlorine with nitrosyl sulfuric acid at high pressure and elevated temperature. Here too, the problem of corrosion leads to the use of expensive materials and hence to high capital costs.
FR 14 97 776 describes a variant of the Deacon process, in which the catalyst is used in the form of a carborundum-CuCl.sub.2 -KCl salt melt (supported liquid phase). At the prevailing reaction temperatures of about 400.degree. C., however, a pronounced discharge of the volatile copper chlorides and hence catalyst losses and contamination of the line sections downstream of the reactor occur in this process.
A further variant of the Deacon process is the Mitsui-Toatsu process, in which HCl is oxidized in a fluidized bed over Cr.sub.2 O.sub.3 /SiO.sub.2 catalysts, the HCl conversion being from 75 to 80%. This process is described in EP 0 184 413, EP 0 277 332, EP 0 331 465, EP 0 465 243 and JP 62 254 846. The disadvantage of this process is the high toxicity of the chromium contained in the Cr.sub.2 O.sub.3 catalysts used.
In the preparation of chlorine by HCl oxidation, a distinction may be made between processes involving a steady-state reaction and those involving a nonsteady-state reaction. In the conventional, steady-state reaction, the HCl-containing feed together with an oxygen-containing gas and possibly further dilution gases is brought into contact continuously as a function of time with the catalyst bed, some of the HCl present being oxidized to Cl.sub.2 and H.sub.2 O, and the reaction products leave the reactor continuously together with unconverted HCl, O.sub.2 and carrier gas. Since the reaction is equilibrium-limited, only partial conversion is possible.
In the nonsteady-state processes known from the prior art, the HCl oxidation is carried out in two steps, the catalyst acting as a material reservoir, or more precisely as a chlorine reservoir. In the loading step, the catalyst is chlorinated with HCl and its oxidic active component phase is converted into a chloride phase and water. After a short flushing phase with inert gas, an oxygen-containing gas flows over the latent catalyst in the second step. Chlorine is liberated and the oxidic phase is formed again.
DE 40 04 454 describes a process for obtaining chlorine by oxidation of HCl over two process stages with the use of a transport catalyst. In the first stage, an HCl gas stream is passed through a fluid bed of copper oxides and NaCl, which are applied to a suitable carrier, and a complex chloride is formed by reaction. After removal of the fluid bed for dechlorination in a second reactor, the oxidized transport catalyst is recycled with injection of O.sub.2 and N.sub.2. U.S. Pat. No. 4,959,202 and EP 04 74 763 likewise describe an unsteady-state process which is carried out in two reactors and in which the HCl loading of the catalyst as well as the dechlorination is effected in the fluidized bed.
WO 91/06505 and U.S. Pat. No. 5,154,911 describe a modified Deacon process with a nonsteady-state reaction with the use of a catalyst which comprises
a) a transition metal oxide selected from MnO.sub.2, Co.sub.2 O.sub.3, Co.sub.3 O.sub.4, Cr.sub.2 O.sub.3, NiO, Ni.sub.2 O.sub.3, Mo.sub.2 O.sub.3, CuO and combinations thereof, PA1 b) an alkali metal chloride, selected from LiCl, NaCl, KCl and combinations thereof, PA1 c) a promoter, selected from LaCl.sub.3, PrCl.sub.3, Pr.sub.2 O.sub.3 and combinations thereof.
The process comprises a chlorination and an oxidation step and is carried out in a fluidized-bed or fixed-bed reactor, and, if required, the catalyst bed may be exchanged between the reaction zones. A similar process for the use of a fixed-bed reactor is described in EP 0 500 728.
DE 43 36 404 likewise describes a modified Deacon process involving a nonsteady-state reaction. It is proposed to dry the HCl gas used by means of a molecular sieve in order to bind the water of reaction formed during the loading phase. Pure oxygen at from 1.0 to 50 bar and from 100 to 500.degree. C. is to be used for the oxidation. The catalysts proposed are manganese oxides and vanadium oxides. The high volatility of vanadyl chlorides and the excessively high activity of MnO.sub.2 are to be regarded as problematic in this process, so that chlorine formation is to be expected as early as during the loading phase. Furthermore, the corrosion resistance of the proposed zeolite molecular sieves is questionable. Owing to the high acidity of the zeolites, it must be assumed that there will be considerable HCl adsorption onto the molecular sieve, which HCl forms hydrochloric acid with water also absorbed and attacks the carrier.
H. Y. Pan, R. G. Minett, S. W. Benson and T. T. Tsotsis, Ind. Eng. Chem. Res. 33 (1994), 2996-3003, describe a reactor concept involving coupling of two alternately operated fluidized-bed reactors, a supported CuCl.sub.2 -NaCl system being used as the catalyst.
The known processes thus have specific disadvantages which arise in particular from incomplete conversion of the hydrogen chloride used and the corrosion problems which therefore arise or the necessity of production processes which employ complicated apparatus and are thus expensive.